1. Field of the Invention
The present invention relates to a system for cold junction compensation for a plurality of thermocouples.
2. Discussion of Prior Art
In some applications, such as temperature measurement in aero engines, the temperatures to be measured are in excess of those which can be tolerated by conventional NiCr/NiAl thermocouples (commonly referred to as K-type thermocouples). The total gas temperature (TGT) inside some new jet engines is above 1200xc2x0 C. which is above the reliable operating limit for a K-type thermocouple. Because of this it is desirable to fabricate thermocouples for such high temperature applications from Platinum and Platinum-Rhodium (Pt/PtRhxe2x80x94commonly referred to as R-type thermocouples although other reference symbols are sometimes used). Such thermocouples have a much higher tolerance to high temperatures.
A thermocouple may be used in the measurement of temperature by measurement of a thermal voltage generated therein which is a function of the difference in temperature between its hot junction (usually located at the place where the temperature is to be measured) and its cold junction (which may be remote from the hot junction and which is at a known temperature).
A disadvantage of using R-type thermocouples is that the materials are relatively expensive. When connection is made to a remotely located cold junction it is practical to use low cost compensating cables to carry the voltage signal to measurement electronics, but only if the temperature is less than 100xc2x0 C. (beyond which such cables are unsuitable). At temperatures above this, which are common in aero engines, it is necessary to use thermocouple wires to carry the voltage signal to the necessary measurement electronics, (situated usually at a safe distance from the extreme temperatures to be measured). In such situations the thermocouple installation becomes unacceptably expensive.
One solution to the problem of cost is to place the cold junction in a mounting head of the thermocouplexe2x80x94a relatively short distance from the hot junction and for example located on the exterior of an aero enginexe2x80x94which might be at a much lower temperature, and to connect the head back to the measurement electronics using inexpensive copper wire. The temperature at the cold junction must then be measured in order to provide a xe2x80x9ccompensation temperaturexe2x80x9dxe2x80x94ie. the thermocouple""s cold junction temperature which can be used, together with the derived voltage signal to calculate the temperature of the hot junction, since the thermally generated voltage in a thermocouple is a function of the difference in temperature between its hot and cold junctions. For measurement of the temperature at the cold junction a conventional K-type thermocouple, or else a resistance temperature detector (RTD), such as a Platinum resistance thermometer (PRT) may be used.
However, some applications require the measurement of an average temperature from a plurality of thermocouple probes. In an aero engine, for example, there may be, typically, 8 to 17 thermocouple probes each of which is likely to contain at least two thermocouples. Since, according to the above-mentioned arrangement, each measurement (R-type) thermocouple has a pair of (e.g. copper) wires from the thermocouple head to the measurement electronics and each head must have at least one (K-type) thermocouple for measurement of the cold junction temperature at the head, which itself has a pair of wires extending back to the measurement electronics, the number of connecting wires becomes substantial. In many applications space is at a premium and it is undesirable to have large bundles of wires conveying the appropriate signals from the thermocouple heads to the measurement electronics.
Accordingly, embodiments of the present invention aim to provide a system for cold junction compensation of multiple thermocouples in which the need for a large quantity of connecting wires is reduced.
The present invention is defined in the attached independent claims, to which reference should now be made. Preferred features may be found in the dependent claims.
Previous and future references in this text to a wire or wires are to be understood as references to a wired signal path. The skilled man would readily and instantly appreciate that a wire may consist of a multistrand wire or several wires connected in series whilst still remaining a single wire or wired signal path.
In one aspect, the invention provides a system for measuring average temperature comprising a plurality of thermocouples each being formed by a pair of thermocouple wires joined together at their first ends to form a hot junction with the other ends forming a cold junction connected to measurement apparatus, wherein the cold junctions of the thermocouples are connected to each other and to the apparatus by signal wire of a different material to that of the thermocouple wires in a manner such that only two wires connect the plurality of connected cold junctions to the measurement apparatus and a signal proportional to the average of the voltages generated by the thermocouples is supplied to the measurement apparatus and the device further comprises a plurality of further temperature sensitive devices one at each of said cold junctions, the further temperature sensitive devices being connected to each other and to the measurement apparatus in a manner such a signal proportional to the average of the signals generated by the further temperature sensitive devices is supplied to the measurement apparatus.
The present invention provides a device whereby one can simultaneously meet the apparently conflicting desiderata of keeping cost down and providing thermocouples able to withstand high operating temperature, whilst also significantly reducing the amount of wiring necessary to connect a number of thermocouples to measurement apparatus.
The invention enables the manufacture at less cost of a device suitable for measuring average temperature in a high temperature environment such as an aero engine and having connections to measurement apparatus or electronics which take up considerably less space in the, say aero engine, than the known devices.
The inventor of the subject invention is the first to appreciate that it is possible to obtain accurate average temperature measurements by determining a value representative of the average voltage generated by a number of thermocouples and compensating for the cold junction temperatures of these thermocouples by using a single compensation value representative of the average temperature of the different thermocouple cold junctions.
As discussed above, in this text and the claims to a wire or wires are to be understood as references to a wired signal path. The skilled man would readily and instantly appreciate that a wire may consist of a multistrand wire or several wires connected in series whilst still remaining a single wire or wired signal path.
The inventor is the first to appreciate that it is not necessary to individually and independently compensate for the cold junction temperature of each thermocouple and that not doing so allows one to significantly reduce the cost and number of required connection wires and still obtain an accurate measurement of average temperature.